1. Technical Field of the Invention
This invention relates generally to light sources in the field of optoelectronics. More particularly, the invention relates to semiconductor laser light sources that includes photoactive material layer(s).
2. Description of the Related Art
Semiconductor laser light sources are key components in the rapidly expanding field of optoelectronics. The properties of high packing density, large scale integration into microchips, and low manufacturing cost make Vertical Cavity Surface Emitting Lasers (VCSELs) in particular uniquely attractive for applications, such as massive parallel computing, interconnects capable of up to THz (Tera (1012)) bandwidth, and optical information storage technologies.
A standard mesa type VCSEL, is shown schematically as a cross section in FIG. 1, and includes a group of n-doped semiconductor segments 1, a group of p-doped semiconductor segments 2, and an active region 3 for light emission. The groups of n-doped and p-doped semiconductor segments, 1 and 2, are each commonly referred to as Bragg mirrors. Bragg mirror 1 has, for example, alternating n-doped semiconductor layers of high refraction index N-AlxGa1-xAs (4, 6, 8,10) and low refraction index N-AlyGa1-yAs (5, 7, 9) formed as a periodic sequence or periodic arrangement of layers. Bragg mirror 2 has, for example, alternating p-doped semiconductor layers of high refraction index P-AlxGa1-xAs (11, 13, 15,17) and low refraction index P-AlyGa1-yAs (12, 14, 16) and a metal conductor layer 18. The active region 3 includes, for example, a layer of N-AlyGa1-yAs 19, one (or more) active layers (Quantum Wells QW) of low band-gap p-GaAs 20, and a layer of P-AlyGa1-yAs 21 formed sequentially and disposed between Bragg mirror 1 and Bragg mirror 2.
In the typical VCSEL, an electron current Je and a hole current Jp flow in opposite directions through n-doped GaAs semiconductor segments of Bragg mirror 1 and p-doped GaAs semiconductor segments of Bragg mirror 2 until they reach the active region (xcex) 3 formed by one or more thin layers of a third semiconductor material sandwiched between the n-doped Bragg mirror 1 and the p-doped Bragg mirror 2. The active region 3 provides light emission via electron-hole pair recombination. The envelope of the radiation profile is characterized by the diameter 2w, w being the radiation waist. The active region 3 material has a smaller energy gap than the semiconductor material of the abutting Bragg mirrors so that (a) the emitted frequency will not be reabsorbed outside the active region and (b) a potential well forms at the p-n junction greatly increasing the carrier density there. The carrier density in the active region, and thus the photon production rate under given external current, increases by orders of magnitude when sub-micron thick active layer structures, known as quantum wells or superlattices are used.
The current-density profile and the light intensity profile in a standard, cylindrical cross-section, single mode VCSEL such as the one shown in FIG. 1, do not match. The fight intensity is peaked at the center of the cross-section (axis), as dictated by the cavity fundamental mode profile, while the current intensity is uniform across the cross section because of the uniform resistivity across the cylindrical VCSEL structure of FIG. 1. The rate of electron-hole recombination, being proportional to the emitted laser light intensity, is therefore higher near the cylinder axis than the rate of replenishment by the current, resulting in carrier depletion in the center of the cylinder.
Central carrier depletion causes undesirable mode switching of the VCSEL. As a consequence of carrier depletion (hole-burning) at the center of the cavity cross-section, currently manufactured VCSELs have a tendency to switch into higher modes at modest output power levels, when the device current is only a few times above threshold (start-up) current. The resulting change in the radiation profile is highly undesirable for a majority of optoelectronic applications.
To prevent that depletion one needs a non-uniform current profile that peaks at the center so as to provide more carriers where the carriers are consumed faster. Increasing the conductivity near the center of the cavity cross-section has been tried to counteract center cavity carrier depletion. Present methods of achieving increased carrier conductivity at the center of the cavity include ion implantation and oxide aperture techniques. Although these techniques are successful in reducing the lasing threshold they still suffer from multi-mode switching at low currents. Thus, these proposed methods do not improve mode switching.
Moreover, ion implantation and oxide aperture techniques have the disadvantage of requiring time consuming and cost increasing wafer post-processing, whereby grown wafers are removed from the growth chamber and subjected to additional processing (exposure to ion bombardment or oxidizing chemical agent). Therefore, the present VCSELs"" do not provide a fabrication process that can attain the low cost associated with standard semiconductor integrated circuit VLSI processing approach.
Another important issue, also stemming from the on-axis carrier depletion, is the appearance of an optical tail after the laser current has been turned off. This is important when VCSELs are employed in producing square optical pulses in digitized optical signals or digital communications. Elimination of imperfections in the optical pulse modulation is crucial in achieving better bit-error-rate (BER); the latter sets a limit on the information transmission rate, and prevents harvesting the full optical fiber communication bandwidth.
One aspect of the invention provides a semiconductor laser with greatly improved laser operation by including photoactive material for self-regulating feedback.
Another aspect of the invention provides a semiconductor laser, for example a VCSEL, with greatly improved laser operation in the fundamental mode, and laser turn-off properties, by using at least one layer of photoactive material. The photoactive layer(s) produces a self-regulating feedback mechanism by inducing a photocurrent that mirrors the fundamental mode intensity and thus produces more carriers on axis to counteract carrier depletion (xe2x80x9chole burningxe2x80x9d).
Consequently one embodiment of the invention enables self-regulated mode control during Vertical Cavity Surface Emitting Laser (VCSEL) operation, preventing mode switching and thus allowing operation at the fundamental laser mode at high radiation power and at much higher device current than the start-up threshold. In addition, the present invention reduces carrier depletion on axis thereby reducing optical tails after laser bias is turned off.
In view of the above, one advantage of the present invention is to provide an improved semiconductor laser, for example a VCSEL, with photocurrent feedback.
Another advantage of the present invention is to improve a high power VCSEL operation in the fundamental cavity mode.
A further advantage of the present invention is to prevent mode switching to undesirable cavity modes in a VCSEL.
A still further advantage of the present invention is to provide a self-regulating feedback mechanism in a VCSEL that does not require outside control circuits or external user intervention.
Another advantage of the present invention is to reduce or eliminate tails in the optical laser pulse after a VCSEL turn-off.
A further advantage of the present invention is to reduce or eliminate power xe2x80x9cspikesxe2x80x9d and pulse modulation/frequency chirp during laser turn-on.
An additional advantage of the present invention is that its reduction to practice is fully compatible with very large scale integration (VLSI), does not require wafer post-processing, and preserves the VCSEL low cost advantage over edge emitting lasers.
A further additional advantage of using thin photoactive layer(s) is that only a small fraction of the circulating cavity power needs to be consumed to induce significant resistivity changes and current channeling so that the VCSEL operating voltage, current, and output power remain close to that found in VCSELs without photo-feedback and the photoactive layer(s) have small fractional absorption with insignificant optical mode distortion.
The photoactive layer may be formed of, for example, high band gap Ga1-xAlxAs, amorphous Ga1-xAlxAs, undoped GaAs, Ge, GeSi, and ZnSe.
As can be appreciated by one skilled in the art, the invention is relevant to semiconductor lasers in general, and more specifically to VCSEL devices, which are formed using semiconductor integrated circuit fabrication methods. However, it is inherent that the invention is also applicable to any semiconductor optics application or device wherein photocurrent feedback can be useful to enable self-regulation of the photo device.